Network Working Group D. Thaler
Internet-Draft Microsoft
Intended status: Informational March 5, 2018
Expires: September 6, 2018
Using URIs With Multiple Protocol Stacks
draft-thaler-appsawg-multi-transport-uris-02
Abstract
Many Uniform Resource Identifiers (URIs) today have some mechanism to
resolve them to one or more specific endpoints where that resource is
available. This document discusses issues that arise when the same
resource can be reached over multiple protocol stacks, and discusses
various approaches that have been used or discussed, and the
tradeoffs between them. Such issues are important to consider when
defining new URI schemes and resolution mechanisms.
Status of This Memo
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This Internet-Draft will expire on September 6, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol endpoint discovery . . . . . . . . . . . . . . . . . 4
3.1. Specified by the URI scheme specification . . . . . . . . 5
3.2. Passed in one URI . . . . . . . . . . . . . . . . . . . . 5
3.3. Use separate URI for each transport endpoint . . . . . . 7
3.4. Use another mechanism for discovery . . . . . . . . . . . 7
4. Transport endpoint selection . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
For Uniform Resource Identifier (URI) schemes that function as
locators (historically called "URLs"), [RFC3986] explains that:
URI "resolution" is the process of determining an access mechanism
and the appropriate parameters necessary to deference a URI; this
resolution may require several iterations. To use that access
mechanism to perform an action on the URI's resource is to
"dereference" the URI.
The specific details vary by URI scheme and hence are up to each URI
scheme definition to specify. Requirements for URI scheme
definitions are covered in [RFC3986], [RFC7320], and [RFC7595]. RFC
7595 section 3.3 states:
For schemes that function as locators, it is important that the
mechanism of resource location be clearly defined.
Closely related to the concept of resolving a URI to a resource that
may have multiple ways to reach it, is the concept of "equivalence".
[RFC3986] section 6.1 states:
Even though it is possible to determine that two URIs are
equivalent, URI comparison is not sufficient to determine whether
two URIs identify different resources. For example, an owner of
two different domain names could decide to serve the same resource
from both, resulting in two different URIs. Therefore, comparison
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methods are designed to minimize false negatives while strictly
avoiding false positives.
Thus, it is possible that two distinct URIs refer to the same
resource. The goal, as RFC 3986 stated above, is simply to
"minimize" such cases, but such minimization often comes at a cost.
For example, for many URIs schemes, a DNS name can be used in the
authority component rather than using several URIs that differ only
in IP address literal, with the cost being a dependency on DNS name
resolution and the potential latency and traffic involved.
As another example, [RFC5630] section 4.1 states:
SIP and SIPS URIs that are identical except for the scheme itself
(e.g., sip:alice@example.com and sips:alice@example.com) refer to
the same resource. This requirement is implicit in [RFC3261],
Section 19.1, which states that "any resource described by a SIP
URI can be 'upgraded' to a SIPS URI by just changing the scheme,
if it is desired to communicate with that resource securely".
This does not mean that the SIPS URI will necessarily be
reachable, in particular, if the proxy cannot establish a secure
connection to a client or another proxy. This does not suggest
either that proxies would arbitrarily "upgrade" SIP URIs to SIPS
URIs when forwarding a request (see Section 5.3). Rather, it
means that when a resource is addressable with SIP, it will also
be addressable with SIPS.
Similarly, the same resource might be identified using both "http"
and "https", and indeed a commonly followed rule (section 4.1.3 of
[USWP]) is that the URI scheme sets expectations for integrity of
access, such that separate integrity levels result in separate URI
schemes.
Thus, the same resource might be identified by multiple URIs that
differ only in URI scheme, or authority component, or path (e.g.,
using ".." resolution).
For URIs used in the World Wide Web, Section 2.3.1 of "Architecture
of the World Wide Web" [AWWW] further discusses such aliasing,
explaining that links to a resource increase the value of that
resource, and multiple URIs for it interfere with such valuation, and
also makes it difficult to correlate two sources as pointing to the
same resource via differing aliases. Thus to maximize the benefit to
the Web, URI aliases should be minimized.
See "URI Schemes and Web Protocols" [USWP] for additional discussion
on the relationship between URI schemes and protocols in a web
context, although that document has no official standing and there is
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a history of difficulty in reaching consensus on the connection
between URI schemes and protocols.[Noah]
2. Problem Statement
Besides specifying one or more URI scheme names to be used and the
syntax for each (e.g., what the authority component contains), there
are two issues a URI scheme definer must deal with when multiple
protocol stacks are available for accessing a given resource:
1. Specifying how the set of protocol endpoint identifiers (e.g.,
TCP and UDP port numbers) for a given URI can be discovered by an
entity wishing to resolve it, and
2. Specifying how an appropriate protocol endpoint can be selected
for use, from among the discovered set.
At a high level, these issues are equivalent to those arising when
multiple IP addresses are available for the same resource. However,
in general, there may be multiple layers in a transport stack (e.g.,
some application-layer protocol over WebSockets over TCP), each with
its own identifiers, so the problems are compounded when multiple
choices exist at each of multiple layers below the application-layer
protocol itself.
Thus, when we use the term "protocol stack" in this document, we
typically mean the stack of protocols below the application-layer
protocol associated with the URI scheme, and above the network layer.
However, [USWP] also discusses the possibility ("Approach 2") that
multiple application-layer protocols might share the same URI scheme,
in which case the "protocol stack" also includes the application-
layer protocols to select from.
3. Protocol endpoint discovery
A client wishing to access a resource needs to know, for each layer
in the protocol stack, what protocol(s) can be used, and what
identifier(s) are needed by each such protocol. There are several
possible approaches to endpoint identifier discovery, which we cover
in the following sections. For simplicity, we will discuss them as
if the same approach is used for both types of information, but it is
important to remember that a URI scheme could specify discovery of
the set of protocols via one approach, and discovery of the
identifier(s) for each protocol via another approach.
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3.1. Specified by the URI scheme specification
In this approach, every resource is assumed to use the exact same set
of transport protocols (i.e., stacks of protocols above the network
layer) and identifiers. The identifiers can be IANA assigned and
specified as part of the URI scheme or protocol specification. For
example, TFTP only supports UDP port 69, and so no port number is
permitted in a tftp URI.
If support for a new transport protocol is later added under a
protocol with a given URI scheme, different entities may thus have
different hard-coded assumptions about the set of possible protocols,
which just pushes the rest of the burden to the problem of selection
among the known set (see Section 4).
A disadvantage of this approach for many use cases is that it does
not allow for non-default server configurations such as custom ports.
3.2. Passed in one URI
For single-transport protocols, a common mechanism is to specify a
default port for the URI scheme, and to allow putting a non-default
port number in the URI authority component.
For multi-transport protocols, historically it was sometimes assumed
that multiple transport protocols (e.g., UDP and TCP) would use the
same port number, so specifying a single number would also be
sufficient for multiple transports. When port numbers appear in
URIs, they are not the default ports that might be IANA-assigned
(since default ports should be omitted from the URI per [RFC3986]
section 3.2.3), but instead are either statically chosen by the
server application, or are ephemeral ports dynamically allocated on
the server hosting the resource. In most TCP/IP stacks, ephemeral
ports used by UDP endpoints have no relationship to ephemeral ports
used by TCP endpoints in the same application and so it cannot be
guaranteed that the port numbers are the same. For example, port
51000 might be allocated to one application for UDP, and a different
application for TCP.
Since 2011, this same issue can also occur with IANA-assigned ports,
especially if support for a given transport protocol is added at a
later time. [RFC6335] section 7.2 explains:
Effective with the publication of this document, IANA will begin
assigning port numbers for only those transport protocols
explicitly included in an assignment request. This ends the long-
standing practice of automatically assigning a port number to an
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application for both TCP and UDP, even if the request is for only
one of these transport protocols.
Thus, for most URI schemes, a port number appearing in a URI
authority component must be specified as being in a specific
transport-layer protocol's numbering space since its value for a
given resource might differ by transport protocol. If a URI scheme
wishes for the port number in the URI authority component to be able
to apply to multiple transport protocols, the URI scheme would
typically have to assume static configuration on servers; this may be
acceptable in some circumstances and unacceptable in others.
A common solution in non-URI contexts is to use a service name rather
than a literal port number, and allow the service name to be resolved
to the relevant transport-layer identifier. Indeed, [RFC6335]
section 3 says:
Because the port number space is finite (and therefore
conservation is an important goal), the alternative of using
service names instead of port numbers is RECOMMENDED whenever
possible.
Unfortunately, it is not possible to follow this recommendation with
the port field in URI authority component, since the URI syntax only
allows integers in the port field.
For new URI schemes, it may be possible in some cases to place a
service name in the host field, such as "_myservice._tcp.example.org"
as would be used with a DNS SRV record [RFC2782]. That example still
specifies only a single transport protocol stack ("_tcp") however,
rather than a list of supported stacks.
Another limitation of service names is that they are currently
limited only to TCP, UDP, SCTP, and DCCP, and so cannot be used with
other layers (e.g., websockets) or protocols. Thus, a URI scheme for
a protocol that supports both, say, websockets and raw TCP as
possible transports for resource access, cannot use a service name as
a common identifier for transport-layer endpoint resolution.
It is usually also undesirable to put transport-layer endpoint
information (the list of supported transport protocols or the
identifier(s) used with the transport protocols) in the path or query
components for two reasons. First, those components are typically
passed over the wire to the server when accessing a resource, which
only consumes extra bandwidth with no benefit. Second, if the
transport-layer identifiers might change over the lifetime of the
resource, then the URI would need to change even if the change did
not affect the actual endpoint chosen by the client. Such a change
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would negatively affect equivalence with the previous URI, e.g.,
resulting in cache misses.
Thus, an advantage of this approach is that it can work without any
dependency on other protocols or deployment of servers needed for
resolution, and a disadvantage is that putting information about
multiple transport-layer endpoints anywhere in the same URI could
make for a very long URI that might have issues with certain
software, or have bandwidth or storage issues.
3.3. Use separate URI for each transport endpoint
In this approach, one must simply accept the fact that multiple URIs
might refer to the same resource as RFC 3986 already allows. This is
similar to using a set of URIs that differ only in IP address
literal, for a case when the resource server is not resolvable via a
protocol such as DNS or SIP.
The obvious disadvantage is that there are multiple URIs for the same
resource. Another potential disadvantage for some more complex use
cases where there are multiple layers of the transport stack, is that
it may be difficult or impossible to express all the identifiers in
an entire stack of protocols in one URI.
For cases where there are multiple transport protocols but only one
such layer, this approach results in needing to identify a single
transport protocol per URI. As discussed in Section 3.2, this often
cannot be put in the authority component and is undesirable to put in
the path or query component. As a result, such cases involve
specifying a separate URI scheme per transport. For example, "sip"
and "sips" do this, as do "http" and "https". RFC 8323 [RFC8323]
also follows this approach for CoAP with "coap", "coaps", "coap+tcp",
"coaps+tcp", etc.
3.4. Use another mechanism for discovery
In this approach, a URI scheme definer would specify a mechanism
whereby transport stack identifiers can be resolved for a given URI,
and the identifiers would come in a form that may not be expressed as
a URI. If multiple layers exist, then such resolution might involve
a resolution step for each layer.
DNS records (e.g., SRV records) provide one potential mechanism that
can be used to discover a set of supported transports and their
associated identifiers. Other types of directories might be usable
in other cases. For example, HTTP now provides an "Alt-Svc"
[RFC7838] mechanism that can discover alternate transport endpoints
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for the same HTTP URI. Another example mentioned in [USWP] is where
the protocol to use is identified by a media type value.
One challenge in many cases is defining a common mechanism that could
discover identifiers for different transport protocols for the same
resource. For example, websockets use URIs and TCP uses port numbers
(and there is currently no URI scheme for TCP itself), and so the
syntax of such identifiers may differ if an application layer
protocol could use both TCP and websockets.
The advantage of requiring a separate resolution mechanism is that
the resource URI itself can be kept short and simple. The downsides
are extra complexity in both clients and servers, potentially extra
specification work for the URI scheme definer, the possible
additional deployment burden of provisioning and operating extra
protocols or servers to facilitate such resolution, and any
additional bandwidth or latency of doing the resolution.
In some contexts, it might be feasible to discover the additional
identifiers using the same mechanism used to discover the URI itself,
perhaps even in the same message.
4. Transport endpoint selection
The URI scheme should specify the mechanism for choosing among
transport protocol stacks, such as specifying at least one that is
mandatory to implement and an algorithm for trying possible transport
stacks in some order until one works. The URI scheme might even
leave it up to the client implementation or client configuration
options as suggested in Approach 2 of [USWP].
The endpoint selection problem is similar to that of choosing among
multiple discovered IP addresses for the same transport stack, and
two common solutions are used today in that context. One category of
algorithm is to sort the choices according to some criteria, and then
to try them in order of preference. For example, SRV records provide
a priority and weight for each transport endpoint that can be used to
sort them, and [RFC6724] provides an algorithm for sorting
destination IP addresses.
Another category of such algorithms is called "Happy Eyeballs"
[RFC6555] where multiple possibilities are attempted in parallel
(possibly with some delay added before starting non-preferred
choices) and keeping the first one that responds successfully. The
advantage is faster connection when a non-preferred choice is needed,
and the disadvantages are extra complexity in the client, extra
traffic on the network, and extra connections at the server if
multiple parallel attempts succeed.
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As noted earlier, when multiple layers exist in the transport stack,
the number of possible permutations might be large in some cases, and
so a mechanism must be cognizant of that.
5. IANA Considerations
This document has no actions for IANA.
6. Security Considerations
The security considerations in section 3.7 of [RFC7595] and section 7
of [RFC3986] apply. [RFC6943] also discusses security considerations
with determining equivalence, and section 3.1.4 of that document is
relevant to resolution. This document does not raise additional
security issues.
7. Acknowledgements
Thanks to Graham Klyne, Alexey Melnikov, and Gabriel Montenegro for
helpful suggestions on this document.
8. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
.
[RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630,
DOI 10.17487/RFC5630, October 2009,
.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
2012, .
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[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, .
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, .
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
.
[AWWW] Jacobs, I. and N. Walsh, "Architecture of the World Wide
Web, Volume One", December 2004,
.
[USWP] Mendelsohn, N., "URI Schemes and Web Protocols", November
2005,
.
[Noah] Mendelsohn, N., "Email from Noah Mendelsohn to the URI-
Review mailing list", July 2017, .
Author's Address
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Dave Thaler
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
Email: dthaler@microsoft.com
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